1. Field of the Invention
The present invention relates to a seek control system for controlling a pickup or read/write head to move to a desired target track on an information storage disc such as a magnetic disc, an optical disc, a magneto-optical disc, or the like.
2. Description of the Prior Art
One known optical disc recording/reproducing apparatus has an optical pickup movable radially over an optical disc under seek control, as disclosed in Japanese Patent Application No. 1-312940. When the optical pickup is to move to a desired target track on the optical disc, the square root of the distance from the present track to a target track is used as a target value, and the difference between the target value and the speed at which the optical pickup is moved by a pickup actuator is detected. The optical pickup is moved to the target track based on the detected difference.
FIGS. 1 and 2 show conventional seek control systems for use in the optical disc recording/reproducing apparatus. FIG. 1 shows a control circuit arrangement for moving an optical pickup by a distance xo which is half a distance 2xo from a starting position to a desired target track. The control circuit arrangement shown in FIG. 1 includes a calculating circuit 1 for calculating the square root .sqroot.x of a distance x that the optical pickup has moved toward the target track. The output signal from the calculating circuit 1 is supplied to a multiplier 2 which multiplies the square root .sqroot.x by a coefficient .sqroot.2 .vertline..alpha..vertline. supplied from a coefficient generator 3. .vertline..alpha..vertline. in the coefficient .sqroot.2 .vertline..alpha..vertline. represents the absolute value of an acceleration .alpha. of the moving optical pickup. The output signal from the multiplier 2 is supplied to a noninverting input terminal of an operational amplifier 4 which serves as a comparator. The operational amplifier 4 has an inverting input terminal that is supplied with a speed x of the moving optical pickup, as detected by a speed detector 5. The operational amplifier 4 determines the difference between the output signal from the multiplier 2 and the speed x of the optical pickup. The operational amplifier 4 applies an output signal to an actuator 6, which then moves the optical pickup based on the applied output signal.
FIG. 2 shows a control circuit arrangement for controlling an optical pickup when the remaining distance to a target track becomes half a distance from a starting position to the target track. The control circuit arrangement shown in FIG. 2 includes a calculating circuit 1' for calculating the square root .sqroot.2xo-x of the difference between a distance 2xo from an original position to a desired target track and a distance x that the optical pickup has moved toward the target track. The output signal from the calculating circuit 1' is supplied to a multiplier 2 which multiplies the square root .sqroot.2xo-x by a coefficient .sqroot.2 .vertline..alpha..vertline. supplied from a coefficient generator 3. The output signal from the multiplier 2 is supplied to a noninverting input terminal of an operational amplifier 4. The operational amplifier 4 determines the difference between the output signal from the multiplier 2 and a speed x of the optical pickup, as detected by a speed detector 5. The operational amplifier 4 applies an output signal to an actuator 6, which then moves the optical pickup based on the applied output signal.
The number of tracks traversed by the optical pickup is used to detect when the remaining distance to the target track becomes half the distance from the starting position to the target track.
A seek control process for the optical pickup is carried out as shown in FIGS. 3A, 3B, and 3C, using the control circuit arrangements shown in FIGS. 1 and 2. FIGS. 3A, 3B, and 3C show, respectively, the acceleration and speed of the optical pickup and the distance traversed by the optical pickup, as they vary with a time T in which the optical pickup moves to the target track. Before the distance traversed by the optical pickup becomes half the distance from the starting position to the target track, the speed of the optical pickup increases with a constant acceleration .alpha., and when the distance traversed by the optical pickup exceeds half the distance from the starting position to the target track, the speed of the optical pickup decreases with an inverted acceleration (deceleration)-.alpha., as shown in FIG. 3A. The optical pickup stops when it reaches the target track. As shown in FIG. 3B, the speed .alpha.t of the optical pickup increases until the distance traversed by the optical pickup becomes half the distance from the starting position to the target track, and decreases when the distance traversed by the optical pickup exceeds half the distance from the starting position to the target track. As shown in FIG. 3C, the distance (1/2).multidot..alpha.t.sup.2 traversed by the optical pickup until the time T elapses varies in a substantially S-shaped pattern. That is, the distance traversed by the optical pickup increases at a maximum rate when the the distance traversed by the optical pickup becomes half the distance from the starting position to the target track, and at a very small rate immediately before the optical pickup reaches the target track.
The above seek control process enables the optical pickup to move to the target track within a short period of time, and hence at a high speed.
With the conventional seek control process, as the optical pickup approaches the target track, the speed thereof is gradually lowered, and when the optical pickup arrives at the target track, the speed thereof becomes zero. Immediately before the optical pickup reaches the target track, the speed thereof is very low, making it difficult to stop the pickup exactly on the target track. More specifically, the speed of the optical pickup may actually become zero immediately before the optical pickup reaches the target track. In such a case, the optical pickup stops when its speed becomes zero, and is positioned off the target track.
The processes for calculating the square roots .sqroot.x, .sqroot.2xo-x with the calculating circuits 1, 1' are very complex and time-consuming, and therefore pose a large burden on the calculating circuits 1, 1'.